Bidirectional bipolar junction transistor operation, circuits, and systems with two base junctions clamped by default

ABSTRACT

Methods, systems, circuits, and devices for power-packet-switching power converters using bidirectional bipolar transistors (BTRANs) for switching. Four-terminal three-layer BTRANs provide substantially identical operation in either direction with forward voltages of less than a diode drop. BTRANs are fully symmetric merged double-base bidirectional bipolar opposite-faced devices which operate under conditions of high non-equilibrium carrier concentration, and which can have surprising synergies when used as bidirectional switches for power-packet-switching power converters. BTRANs are driven into a state of high carrier concentration, making the on-state voltage drop very low.

CROSS-REFERENCE

Priority is claimed from U.S. applications 61/838,578 filed Jun. 24,2013; 61/841,624 filed Jul. 1, 2013; 61/914,491 filed Dec. 11, 2013;61/914,538 filed Dec. 11, 2013; 61/924,884 filed Jan. 8, 2014;61/925,311 filed Jan. 9, 2014; 61/928,133 filed Jan. 16, 2014;61/928,644 filed Jan. 17, 2014; 61/929,731 filed Jan. 21, 2014;61/929,874 filed Jan. 21, 2014; 61/933,442 filed Jan. 30, 2014;62/007,004 filed Jun. 3, 2014; and 62/008,275 filed Jun. 5, 2014, eachand every one of which is hereby incorporated by reference.

BACKGROUND

The present application relates to bidirectional bipolar transistors,and more particularly to power converters which incorporatebidirectional bipolar transistors, and also to related methods.

Note that the points discussed below may reflect the hindsight gainedfrom the disclosed inventions, and are not necessarily admitted to beprior art.

Power Packet Switching Converters

A new kind of power converter was disclosed in U.S. Pat. No. 7,599,196entitled “Universal power conversion methods,” which is incorporated byreference into the present application in its entirety. This patentdescribes a bidirectional (or multidirectional) power converter whichpumps power into and out of a link inductor which is shunted by acapacitor.

The switch arrays at the ports are operated to achieve zero-voltageswitching by totally isolating the link inductor+capacitor combinationat times when its voltage is desired to be changed. (When theinductor+capacitor combination is isolated at such times, the inductor'scurrent will change the voltage of the capacitor, as in a resonantcircuit. This can even change the sign of the voltage, without loss ofenergy.) This architecture is now referred to as a “current-modulating”or “Power Packet Switching” architecture. Bidirectional power switchesare used to provide a full bipolar (reversible) connection from each ofmultiple lines, at each port, to the rails, i.e. the internal linesacross which the link inductor and its capacitor are connected.

The conventional epitaxial base NPN transistor has an N+ region over theentire back surface. This necessarily prevents the structure from havingthe same electrical characteristics in each direction when operated as abidirectional transistor. These structures are therefore not well-suitedto acting as bidirectional switches in power-packet-switching powerconverter architectures.

Semiconductor statistics under high level non-equilibrium carrierdensities can be quite different from low level carrier densities.Conventional recombination is generally less relevant with high levelcarrier density than in low level conditions. Typical definitions ofcarrier lifetime are also less relevant. Carriers can often interactdirectly in high level conditions through Auger interactions. The beta(ratio of emitter current to base current) will therefore normallydecrease as a bipolar transistor is driven into high levelnon-equilibrium carrier densities. These densities can be, for example,more than two orders of magnitude above intrinsic carrier density.

The voltage drop under conditions of high level non-equilibrium carrierconcentration will be low, even if the resistivity under small currentsis large. Thus a device can be optimized to withstand high voltages(e.g. 1200V or more) while still achieving a forward voltage drop ofless than a Volt.

The present application teaches, among other innovations,power-packet-switching power converters in which bidirectional bipolartransistors are used as switches.

The present application also teaches, among other innovations, methodsfor operating power-packet-switching power converters usingbidirectional bipolar transistors for fully-bidirectional switching.

The present application also teaches, among other innovations,power-packet-switching power converters in which drive circuits operatebidirectional bipolar transistors for bidirectional switching.

The present application also teaches, among other innovations, methodsfor operating power-packet-switching power converters using drivecircuits to control bidirectional bipolar transistors for bidirectionalswitching.

The present application also teaches, among other innovations, methodsfor fabricating bidirectional bipolar transistors forpower-packet-switching power converters.

The present application also teaches, among other innovations,power-packet-switching power conversion systems having bidirectionalbipolar transistors, in which power-packet-switching power convertersuse bidirectional bipolar transistors for switching.

The present application also teaches, among other innovations, methodsof operating power-packet-switching power conversion systems, in whichbidirectional bipolar transistors are used for bidirectional switching.

The above innovations are implemented, in various disclosed embodiments,by using merged double-base bidirectional opposite-faced devices whichoperate under conditions of high non-equilibrium carrier concentration,and which avoid diode drops. For maximum efficiency, it is preferable touse devices which provide bidirectional conduction with less than adiode drop (approximately 1 V in silicon) in each direction, despitereduced effective gains.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed inventions will be described with reference to theaccompanying drawings, which show important sample embodiments and whichare incorporated in the specification hereof by reference, wherein:

FIGS. 1A and 1B show sample embodiments of devices according to thepresent inventions.

FIG. 2 shows one sample embodiment of a base drive circuit according tothe present inventions.

FIGS. 3A, 3B, 3C, 3D, 3E, and 3F show sample equivalent circuits for anexemplary device in various stages of operation.

FIG. 4A shows a sample embodiment of a device according to the presentinventions.

FIG. 4B shows a sample conventional transistor.

FIG. 5A shows a sample proposed circuit symbol.

FIG. 5B shows a circuit symbol for a conventional transistor.

FIG. 6 shows another sample embodiment of the present inventions.

FIG. 7A shows a top view of a sample conventional transistor.

FIG. 7B shows a top view of one sample embodiment of the presentinventions.

FIGS. 8A, 8B, 8C, and 8D show several sample embodiments of the presentinventions.

FIG. 9 shows one sample embodiment of a base drive circuit according tothe present inventions.

FIG. 10 shows sample currents and voltages for one sample embodiment ofthe present inventions.

FIG. 11 shows another sample embodiment of a base drive circuit.

FIG. 12 shows another sample embodiment of a base drive circuit.

FIGS. 13A, 13B, and 13C show further sample embodiments of devicesaccording to the present inventions.

FIGS. 14A-14B show further sample embodiments of devices according tothe present inventions.

FIG. 15 shows another sample embodiment of a device according to thepresent inventions.

FIGS. 16A, 16B, 16C, 16D, and 16E show one sample process forfabricating a device according to the present inventions.

FIG. 17 shows a sample pad mask for fabricating a device according tothe present inventions.

FIGS. 18A-18B show another sample embodiment of a pad mask forfabricating a device according to the present inventions.

FIG. 19 shows one sample embodiment of a termination structure for adevice according to the present inventions.

FIGS. 20A, 20B, 20C, and 20D show sample embodiments of pad arrangementsaccording to the present inventions.

DETAILED DESCRIPTION OF SAMPLE EMBODIMENTS

The numerous innovative teachings of the present application will bedescribed with particular reference to presently preferred embodiments(by way of example, and not of limitation). The present applicationdescribes several inventions, and none of the statements below should betaken as limiting the claims generally.

An important general principle of the various inventive embodimentsdisclosed herein is that, in a power-packet-switching power converter,switching is accomplished synergistically by using merged double-basebidirectional opposite-faced devices which operate under conditions ofhigh non-equilibrium carrier concentration. For maximum efficiency, itis preferable to use devices which provide bidirectional conduction withless than a diode drop (approximately 1 V in silicon) in each direction.

A power-packet-switching (PPS) converter needs fully bidirectional drivedevices, due to the unique architecture. Efficiency is a key criterionin PPS converter designs—even more than in other converter designs,because PPS converters already achieve such high efficiencies. (Anadditional 1% of inefficiency is minor in a design which operates ate.g. 85% efficiency, but makes a drastic difference in a design which isachieving 98%.)

One element of efficiency is losses in the switching devices. IGBTdevices are inherently liable to an on-state voltage difference causedby the forward “diode drop” voltage (in addition to any resistivelosses). While FETs typically do not suffer from a diode drop, they mustdeal with the lack of conductivity modulation: their ON resistancecannot be lower than the intrinsic resistance of the base semiconductor.

The innovative devices of the present inventions overcome the diode dropproblem of the IGBT, while also achieving very low ON resistance fromconductivity modulation. Switching losses are low, at least in part byvirtue of active turn-off from having base contacts on both sides. IGBTshave inherently slow turn-off since they turn-off open base, whichforces those devices to be made with very short carrier lifetimes, whichin turn increases the forward voltage drop.

The present application teaches a new approach to switching devicedesign for PPS converters. By driving a fully-symmetric double-basebipolar into a state of high carrier concentration, the on-state voltagedrop is made very low. This is contrary to conventional wisdom, becausesuch a heavy drive current imposes greater demands on the base drivecircuitry, and reduces the effective gain (beta) of the device.

The fully-symmetric double-base bipolar is implemented as a“collectorless” structure, in which emitter and base structures areformed on both front and back surfaces of a semiconductor wafer(typically a thinned wafer). Depending on the instantaneous direction ofcurrent, one of the two opposed emitter diffusions will operate as acollector. Heavy base current is applied to the emitter which is notacting as a collector, to achieve a high non equilibrium carrierconcentration, and hence a low on-state voltage drop.

A number of alternative and improved device structures are disclosed,which individually and in various combinations reduce the required basedrive (for a given power level). These include e.g. heterojunctionemitters, tunnel oxides, field-shaping regions below the base contact,and vertical relations between the emitter and base depths.

The present inventions teach, inter alia, a bidirectional bipolartransistor (BTRAN), which is a vertically-symmetric, four-terminal,three-layer, vertical-current-flow semiconductor device. A BTRAN is mostpreferably formed as an NPN device, but can also be e.g. a PNP device.

BTRANs are high-level injection devices, and recombination thus occursquite differently from in low level injection devices.Presently-preferred sample embodiments combine high level carrierinjection with thick base regions. In one sample embodiment, the baseregion can be e.g. 60 μm.

An important realization is that the voltage drop under conditions ofhigh level non-equilibrium carrier concentration will be low, even ifthe resistivity under small currents is large. Thus a device can beoptimized to withstand high voltages (e.g. 1200V or more) while stillachieving a forward voltage drop which is less than one diode drop.

The disclosed devices combine synergistically with thepower-packet-switching (PPS) power converter architecture by providingfully-bidirectional switching with less than a diode drop, high voltageresistance, and high ruggedness. The disclosed devices can be 100%bidirectional, with low on-resistance at full current.

For reference, in this type of switching bidirectional bipolar, the sidewhich is instantaneously acting as the emitter can be referred to as theanode, and the other side the cathode.

Basic Implementations

Simple NPN and PNP implementations will be described first. Improvementson this generation of sample embodiments, as discussed below, are morepreferable, but this version helps to illustrate basic concepts andprinciples more clearly.

In the NPN sample embodiment of FIG. 1A, p-type semiconductor base layer102A has N+ emitter/collector regions 104A on an upper and a lowersurface. Emitter/collector terminals 106A and 106B are connected onopposite sides of the BTRAN to respective N+ emitter/collector regions104A. Similarly, base terminals 108A and 108B are connected on oppositesides of the BTRAN to respective external portions of P type base/driftregion 102A. Each of base terminals 108A and 108B can be left “open”(e.g. not connected to anything), shorted to respective terminal 106A or106B (e.g. 108A to 106A or 108B to 106B), or connected to a powersource.

The PNP sample embodiment of FIG. 1B is similar to the NPN sampleembodiment of FIG. 1A, except that N-type emitter/collector regions 104Abecome P-type emitter/collector regions 104B, and P-type drift region102A becomes N-type drift region 102B.

Methods of Operation

FIG. 2 shows a simplified schematic of one implementation of a basedrive circuit, which can be used with FIGS. 3A-3F to illustrate thebasic operation of BTRAN 210.

FIG. 3A shows a sample equivalent circuit for one exemplary NPN BTRAN.Body diodes 312A and 312B can correspond to e.g. the upper and lower P-Njunctions, respectively. In, for example, the sample embodiment of FIG.1A, these can correspond to the junctions between emitter/collectorregions 104A and base regions 102A. Switches 314A and 314B can shortrespective base terminals 108A and 108B to respective emitter/collectorterminals 106A and 106B.

In one sample embodiment, a BTRAN can have six phases of operation ineach direction, as follows.

1) Initially, as seen in FIG. 3B, voltage on emitter/collector terminalT1 is positive with respect to emitter/collector terminal T2. Switches314A and 316A are open, leaving base terminal B1 open. Switch 314B isclosed, shorting base terminal B2 to emitter/collector terminal T2.This, in turn, functionally bypasses body diode 312B. In this state, thedevice is turned off. No current will flow in this state, due to thereverse-biased P-N junction (represented by body diode 312A) at theupper side of the device.

2) As seen in FIG. 3C, the voltage on emitter/collector terminal T1 isbrought negative with respect to emitter/collector terminal T2. P-Ndiode junction 312A is now forward biased, and now begins injectingelectrons into the drift region. Current flows as for a forward-biaseddiode.

After a short time, e.g. a few microseconds, the drift layer iswell-charged. The forward voltage drop is low, but greater in magnitudethan 0.7 V (a typical silicon diode voltage drop). In one sampleembodiment, a typical forward voltage drop (Vf) at a typical currentdensity of e.g. 200 A/cm² can have a magnitude of e.g. 1.0 V.

3) To further reduce forward voltage drop Vf, the conductivity of thedrift region is increased, as in e.g. FIG. 3D. To inject more chargecarriers (here, holes) into the drift region, thereby increasing itsconductivity and decreasing forward voltage drop Vf, base terminal B2 isdisconnected from terminal T2 by opening switch 314B. Base terminal B2is then connected to a source of positive charge by switch 316B. In onesample embodiment, the source of positive charge can be, e.g., acapacitor charged to +1.5 VDC. As a result, a surge current will flowinto the drift region, thus injecting holes. This will in turn causeupper P-N diode junction 312A to inject even more electrons into thedrift region. This significantly increases the conductivity of the driftregion and decreases forward voltage drop Vf to e.g. 0.1-0.2 V, placingthe device into saturation.

4) Continuing in the sample embodiment of FIG. 3D, current continuouslyflows into the drift region through base terminal B2 to maintain a lowforward voltage drop Vf. The necessary current magnitude is determinedby, e.g., the gain of equivalent NPN transistor 318. As the device isbeing driven in a high level injection regime, this gain is determinedby high level recombination factors such as e.g. surface recombinationvelocity, rather than by low-level-regime factors such as thickness of,and carrier lifetime within, the base/drift region.

5) To turn the device off, as in e.g. FIG. 3E, base terminal B2 isdisconnected from the positive power supply and connected instead toemitter terminal T2, opening switch 316B and closing switch 314B. Thiscauses a large current to flow out of the drift region, which in turnrapidly takes the device out of saturation. Closing switch 314A connectsbase terminal B1 to collector terminal T1, stopping electron injectionat upper P-N junction 312A. Both of these actions rapidly remove chargecarriers from the drift region while only slightly increasing forwardvoltage drop Vf. As both base terminals are shorted to the respectiveemitter/collector terminals by switches 314A and 314B, body diodes 312Aand 312B are both functionally bypassed.

6) Finally, at an optimum time (which can be e.g. nominally 2 μs for a1200 V device), full turn-off can occur, as seen in e.g. FIG. 3F. Fullturn-off can begin by opening switch 314B, disconnecting base terminalB2 from corresponding terminal T2. This causes a depletion region toform from lower P-N diode junction 312B as it goes into reverse bias.Any remaining charge carriers recombine, or are collected at the upperbase. The device stops conducting and blocks forward voltage.

The procedure of steps 1-6 can, when modified appropriately, used tooperate the device in the opposite direction. Steps 1-6 can also bemodified to operate a PNP BTRAN (e.g. by inverting all relevantpolarities).

BTRAN with Deep Emitters on Both Faces

FIG. 4A shows another sample embodiment of an epitaxial base BTRAN (thecircuit symbol of which can be seen in FIG. 5A) according to the presentinventions. By contrast, FIG. 4B shows a conventional epitaxial basebipolar transistor (the circuit symbol of which can be seen in FIG. 5B).These two devices differ significantly in their structures, even beyondthe presence of a second base contact region on the bottom surface ofthe BTRAN.

In the sample embodiment of FIG. 6, base contact regions 636 are notconnected to adjacent heavily doped N+ emitter/collector regions 604 anda relatively high reverse bias is applied across the nearbyreverse-biased collector-to-base junction between emitter/collectorregions 604 and base 602. The depletion region of the reverse-biasedcollector-to-base junction will electrically isolate lower base contactregions 636 from the rest of base 602.

This condition, that base contact regions 636 on one side of the deviceare electrically isolated from the remainder of base 602 when asufficiently high reverse voltage is present across the associatedreverse-biased base-to-collector junction, can be obtained, for example,through a combination of the following parameters.

1) Have a sufficiently-lightly-doped base region. This requirement canbe easily met, since the doping concentration of the base region helpsdetermine the breakdown voltage of the base-to-collector junction, andsince the N+ emitter/collector regions are more heavily doped (as seenin e.g. FIG. 4A).

2) Have N+ regions extend deeper than P+ base contact regions so thedepletion of the reverse-biased base-to-collector junction spreadsacross the region beneath the base contact. This condition can be met,e.g., by introducing the P+ doping species sufficiently long after theintroduction of the N+ doping species, or by using a P+ doping speciesthat diffuses more slowly than the N+ doping species, or by acombination of these two techniques.

3) Use a cell geometry wherein the depletion region of thereverse-biased base-to-collector junction isolates all of the associatedbase contact regions. This requirement is not met in a conventionalepitaxial base NPN transistor, a top view of which can be seen in FIG.7A. For greatest current density, base contact regions will be presenton both sides of each of the emitter regions in the device of FIG. 7A.However, the exemplary BTRAN of FIG. 7B has openings in the N+ emitterregion where P+ base contact regions are formed. Heavily-doped N+emitter regions surround each P+ base contact region with a junctionthat can form the needed depletion region, as seen in e.g. FIGS. 8A-8D.

In FIG. 8A, a low reverse bias is present across terminals B2 and E2/C2.Separate depletion regions have formed around each reverse-biased N+region 804. As the reverse bias increases, the depletion regions beginto merge, as in e.g. FIG. 8B. As the reverse bias continues to increase,the depletion region widens, as in e.g. FIG. 8C. In FIG. 8D, thereverse-bias voltage between emitter/collector terminal E2/C2 and baseterminal B2 has continued increasing until it approaches the breakdownvoltage.

Trench Isolation

Note that FIG. 13A shows oxide-filled trenches, on each surface, whichlaterally separate the emitter region (N+ for an NPN) from the adjacentbase contact region. This structure, and its advantages, will bedescribed further in the following sections.

Tunnel Oxide

As a result of the high level non-equilibrium carrier densities, in anNPN BTRAN, it is typically desirable to maximize electron injection fromemitter to base while minimizing hole injection from base to emitter. Insome sample embodiments, high electron injection from emitter to baseand low hole injection from base to emitter can be achieved using tunneloxide, which can be on the order of e.g. 10 Å (1 nm). Electrons willtypically have a much higher probability to tunnel through the thintunnel oxide than will holes.

In one sample embodiment, this can be accomplished by providing a thinlayer of tunnel oxide between emitter regions and emitter contacts. Inthe sample embodiment of FIG. 13A, a thin layer of tunnel oxide 1324 ispresent between N+ emitter region 1304 and polysilicon layer 1322. Polylayer 1322, in turn, contacts emitter metallization 1326. Oxide 1328fills trench 1330, in addition to separating metallization 1326 and poly1322 from base poly layer 1332 and metallization 1334. Oxide 1328 intrench 1330 further minimizes unwanted same-side carrier recombinationbetween emitter region 1304 and otherwise-adjacent base contact region1336.

In the sample embodiment of FIG. 13B, N+ emitter region 1304B issignificantly smaller than in, e.g., the sample embodiment of FIG. 13A.This in combination with tunnel oxide layer 1324 can reduceopportunities for undesired hole injection and recombination in emitterregion 1304.

The N+ emitter regions can also be entirely absent, as in the sampleembodiment of FIG. 13C. Poly region 1322B can instead act as an emitter,and is protected from undesired hole injection and recombination bytunnel oxide layer 1324 between base region 102A and emitter 1322B. (Itwill of course be understood that the sample embodiments of FIGS.13B-13C show only one side of each device, which will generally befabricated identically on both surfaces.)

Surface recombination velocity, which can be a factor inhigh-level-regime current gain, can be problematic due to recombinationat the metal contacts, primarily on the emitter contacts. In some sampleembodiments, tunnel oxide can be used to nearly eliminate thisrecombination in NPN BTRANs by blocking holes from reaching the emittercontact. In the sample embodiment of FIG. 14A, tunnel oxide 1424A isdisposed between emitter regions 1404 and base 102A. As in FIG. 13A,oxide-filled trench 1330 is disposed between N+ emitter regions 1404 andP+ base contact regions 1336. Since N+ emitter regions 1404 aredeposited on top of tunnel oxide 1424A, N+ regions 1404 can be e.g.polycrystalline silicon, rather than being formed as part of the monocrystalline substrate.

In the sample embodiment of FIG. 14B, tunnel oxide 1424B continues alongthe edge of polycrystalline N+ emitter regions 1404 to the surface ofthe device. In the absence of the oxide-filled trenches of e.g. FIG.14A, P+ base contact regions 1436 are offset from emitter regions 1404in order to not contact tunnel oxide 1424B around N+ emitter regions1404. This separation fulfills a purpose similar to the oxide-filledtrenches of FIG. 14A by minimizing undesirable direct electrical contactbetween base contact regions 1436 and emitter regions 1404. This canhelp minimize unwanted same-side carrier flow and recombination betweenemitter and base contact regions.

The sample embodiment of FIG. 14B is somewhat less preferable than thatof FIG. 14A, however. In the sample embodiment of FIG. 14A, thestructure with sidewall 1330 can be more compact, and tunnel oxide 1424Ais only on the bottom of N+ regions 1404. In the sample embodiment ofFIG. 14B, the necessary separation between N+ regions 1404 and P+regions 1436 increases cell size. Tunnel oxide 1424B must also be formedover a larger surface area than in FIG. 14A, increasing fabricationcomplexity.

Devices with Heterojunction Emitter

In other sample embodiments, electron-hole discrimination can beachieved using heterojunction emitter regions. In the sample embodimentof FIG. 15, emitter regions 1504 can be e.g. amorphous silicon on acrystalline substrate. As amorphous silicon has a band gap of 1.4 V,compared to a band gap of 1.1 V for crystalline silicon, electronsinjected from emitter 1504 to base 102A are relatively energetic, andmore electrons are injected from emitter 1504 to base 102A than holesfrom base 102A to emitter 1504.

Base Drive

Base drive circuits according to the present inventions are preferablyapplied to each of the two base terminals of a BTRAN to permit operationas described herein. In one sample embodiment, sample BTRAN base drivecircuits preferably permit a BTRAN to turn on as a diode; transition toa very-low-forward-voltage saturation mode after initial turn-on;transition back to a no-longer-saturated-but-still-on state; achievestored charge reduction prior to turn-off to reduce tail current; andthen achieve full turn-off and block forward voltage.

In one sample NPN embodiment, a base drive circuit like that of FIG. 9can be applied to both base B1 and base B2, and can operate e.g. asfollows.

-   -   a. Open base B1: base terminal B1 is free to float, and negative        voltage from opposite terminal T2 is blocked. The expected        floating base voltage can be, e.g., in the range of 0.7 V to        less than 20 V. In this state, opposite base terminal B2 is        shorted to respective opposite terminal T2. In the sample        embodiment of FIG. 9, base B1 can be left floating by turning        off MOSFET switches S1 and S2.    -   b. Base B1 shorted to terminal T1: base terminal B1 is shorted        to respective terminal T1, e.g. by turning switch S1 off and        turning switch S2 on. Opposite base terminal B2 is open, and        positive voltage from opposite terminal T2 is blocked.        Alternatively, the base drive can be in this state while the        BTRAN conducts in forward-biased diode mode when opposite        terminal T2 is negative. In this latter state, the nominal        forward voltage drop Vf can be, e.g., between 1 V and 3 V for        silicon diodes.    -   c. Base B1 operatively connected to positive bias: Base terminal        B1 is connected to a source of positive charge, e.g. by turning        on switch S1 and turning off switch S2. In this state, opposite        base B2 is open, while the BTRAN is conducting in forward-biased        saturated NPN bipolar transistor mode. The nominal forward        voltage drop can be, e.g., approximately 0.2 V. A large current        flows to base B1 from the positive bias immediately after        connection. Subsequent current flow is lower.    -   d. Immediately following step (c), base terminal B1 is connected        to respective terminal T1, e.g. by opening switch S1 and closing        switch S2, while opposite base terminal B2 is open. A large        current briefly flows to terminal T1 from base terminal B1. This        rapidly depletes charge carriers in the drift region. The device        comes out of saturation and reverts to forward-biased diode        mode.    -   e. Immediately following step (d), opposite base terminal B2 is        shorted to respective opposite terminal T2 by a similar        mechanism. A small current flows to base terminal B1 from        respective terminal T1, and charge carriers are swept from the        drift region, increasing forward voltage drop Vf.    -   f. Immediately following step (e), base terminal B1 is floated,        e.g. by turning switches S1 and S2 off, while opposite base        terminal B2 remains shorted to respective opposite terminal T2.        The BTRAN turns off as the PN junction between base terminal B1        and respective terminal T1 becomes reverse biased.

In one sample embodiment, switch S2 can be e.g. a GaN MOSFET. Sinceswitch S2 conducts and blocks voltage in both directions, and thelargest positive voltage switch S2 sees is +1.5 V, a GaN MOSFET can bepreferable for switch S2, since the body diode of a GaN MOSFET does notconduct current at 1.5 V or less.

FIG. 10 shows plots of some sample currents and voltages during aprocess according to the present inventions, e.g. like that of steps(a)-(f) above, modified for a PNP sample embodiment like that of FIG.11.

FIG. 11 shows one sample embodiment of a PNP BTRAN base drive circuitwhich can include common source MOSFET pairs for base-emitter shorting.Isolated power supplies P1 and P2 are included in parallel withrespective capacitors C1 and C2. In one sample embodiment, isolatedpower supply P1 can be at, e.g., −0.7 V with respect to emitter E1, andisolated power supply P2 can be at, e.g., −0.7 V with respect to emitterE2.

Common source MOSFET pairs (Q11, Q21) and (Q22, Q12) are preferably usedfor base-emitter shorting between respective emitter-base pairs (E1, B1)and (E2, B2).

JFETs Q31 and Q32 are preferably used at startup to increase blockingvoltage, and then are preferably turned off while the converter isrunning.

MOSFETs Q41 and Q42 are preferably used after device turn-on to reduceforward voltage drop Vf.

FIG. 12 shows a presently-preferred sample embodiment of a base drivecircuit for NPN BTRAN 1210. Two common grounds are shown. Common ground1222 connects together emitter/collector terminal T1 and the base drivecircuitry driving base B1 108A. Common ground 1220 connects togetheremitter/collector terminal T2 and the base drive circuitry driving baseB2 108B.

In one sample embodiment, this base drive circuitry can drive baseterminal B2 108B in one of three modes. In passive off mode, baseterminal B2 108B is preferably clamped by a voltage-limiting device(e.g. by Schottky diode D₂₂) to be no higher than e.g. about 0.3Vrelative to emitter 106B, and is allowed to be lower in voltage thanemitter 106B. In passive off mode, only normally-on JFET S₅₂ is on. Thiscan permit base B2 108B to float, as in e.g. FIG. 3F.

In base-emitter shorting mode, only MOSFET switches S₄₂ and S₃₂ are on,thereby shorting base B2 108B to emitter T2 106B.

Injection mode, for NPN BTRANs, injects current into the respective baseterminal when the device is in active on mode. This can lower theforward voltage drop of the BTRAN to less than a diode drop, e.g. 0.7volts. In some sample embodiments, this step can lower the forwardvoltage drop Vf to e.g. 0.1-0.2 V. Switch S₄₂ is on, while switches S₃₂and S₅₂ are off MOSFET switches S₁₂ and S₂₂ are controlled on and off toproduce an appropriate current into base terminal B2 in switch modepower supply configuration. Current swings can be controlled byinductors L11 and L12, and current can be sensed by resistors R11 andR12. A suitable control system (not shown) controls the switches,inductors, and resistors in order to control the base current, tothereby produce a low forward voltage drop Vf.

Fabrication

In one sample embodiment, the deposition of hydrogenated amorphoussilicon (a-Si:H) or hydrogenated amorphous silicon carbide (a-SiC:H) canadvantageously be used for fabricating BTRANs having heterojunctionemitters as described herein. This material can be sputtered, but ismore preferably deposited using chemical vapor deposition (CVD) orplasma enhanced CVD. Since these materials are altered by hightemperature processing, the emitter material needs to be deposited afterthe high temperature processing steps have been completed.

In one sample embodiment, tunnel oxide can be fabricated between baseand emitter regions as described herein. The bare surface of the baseregion can be exposed to an oxidizing ambient long enough to grow a thinlayer of a dielectric that is primarily e.g. silicon dioxide. In onesample embodiment, this thin layer of tunnel oxide can be e.g. in therange of 10 Å-30 Å. Following such an oxidation step, a layer ofamorphous or polycrystalline silicon can be deposited, e.g. using lowpressure chemical vapor deposition (LPCVD). In some sample embodiments,this can be followed by introducing a dopant such as e.g. arsenic todope the polycrystalline silicon.

The present inventors have realized that physical and electricalperformances can be made nearly equivalent on both sides of a BTRANsemiconductor die. All dopant species are introduced into each side ofthe wafer, and then a single long high-temperature diffusion step ispreferably performed.

In one sample embodiment, the long high-temperature diffusion step canbe, e.g., at a temperature of 1100-1150° C. Long high-temperaturediffusion processes can most preferably be used in conjunction with thetwo handle-wafer process described below, but can also be usedindependently of the two handle-wafer process.

Presently-preferred processing steps for fabricating a BTRAN accordingto the present inventions include masking operations, thermal oxidation,etching, impurity introduction, chemical vapor deposition (CVD), andphysical vapor deposition (PVD).

A sample fabrication sequence according to the present inventions mostpreferably includes both a high-temperature-handle-wafer bonding stepand a medium-temperature-handle-wafer bonding step. These two handlewafers are preferably attached to different sides of the same wafer atdifferent points in the fabrication sequence, and preferably indifferent temperature ranges.

“High temperature”, in the context of the present innovative processes,can mean, e.g., any temperature above the alloy/anneal temperature ofaluminum or aluminum alloys. In one sample embodiment, “hightemperature” can refer to, e.g., any temperature above approximately450° C.

“Medium temperature”, in the context of the present innovativeprocesses, can mean, e.g., any temperature between the meltingtemperature of solder and the alloying temperature of aluminum oraluminum alloys (inclusive). In one sample embodiment, “mediumtemperature” can refer to, e.g., any temperature between approximately240° C. and approximately 450° C. (inclusive).

“Low temperature”, in the context of the present innovative processes,can mean, e.g., any temperature between approximately room temperatureand the melting temperature of solder. In one sample embodiment, “lowtemperature” can refer to, e.g., any temperature between approximately25° C. and 240° C.

In one sample embodiment, a BTRAN fabrication sequence according to thepresent innovative processes can proceed e.g. as follows:

Step 1 through Step M: Perform all high temperature steps, such asthermal oxidation, some chemical vapor deposition (CVD) operations, andhigh temperature anneals, up to the contact mask step, on one side ofthe wafer. This stops just short of a high-temperature, relatively-longdiffusion step designed to diffuse impurities to a desired junctiondepth on both sides of the wafer.

Step M+1: Deposit a protective layer (e.g. CVD silicon dioxide) orsandwich of protective layers (e.g. CVD silicon dioxide, CVD siliconnitride, and CVD silicon dioxide) on the first side of the device wafer.The protective layer can prevent unwanted changes to the first side.This protective layer can also serve as a stopping point for a thinningoperation later used to remove material from the first side.Chemical-mechanical planarization (CMP) can be performed to flatten thetop surface of the deposited protective layer (or deposited sandwich ofprotective layers). A sample of the device at this point can be seen in,e.g., FIG. 16A.

Step M+2: Attach handle wafer 1 to the first side of the device wafer athigh temperature. Any acceptable high-temperature material can be usedfor handle wafer 1, such as e.g. silicon, silicon dioxide, siliconcarbide, or sapphire. Handle wafer 1 must bond to the first side of thedevice wafer. If silicon is used as the handle wafer, the surface of thesilicon handle wafer will bond directly to the top of the protectivelayer or protective layer sandwich at high temperature, as seen in e.g.FIG. 16B.

Step M+3: From a second side opposite the first side, thin the devicewafer to the desired thickness. This can be done, e.g., by a combinationof grinding, lapping, and polishing.

Step M+4 through Step N: Perform steps 1 through M on the second side ofthe device wafer.

Step N+1: Perform a relatively long high-temperature diffusion step toobtain the desired dopant junction depths and dopant distribution onboth sides of the device wafer.

Step N+2 through Step P: Perform steps from contact mask throughpassivation layer deposition and a pad etch step on the second side ofthe device wafer.

Step P+1: Attach handle wafer 2 to the second side of the device waferat a medium temperature. Handle wafer 2 can be any acceptablemedium-temperature material, such as e.g. quartz, glass, silicon,silicon carbide, and sapphire. In one sample embodiment, the device atthis point can be like that seen in e.g. FIG. 16C.

Step P+2: Remove handle wafer 1. This can be done, e.g., by grinding,lapping, chemical-mechanical planarization (CMP), and is most preferablycontinued up to, but not through, the “stopping point” layer depositedin e.g. Step M+1. As a result of this step, the device wafer can be likethat seen in, e.g., FIG. 16D.

Step P+3: Remove the stopping layer (e.g. by etching orchemical-mechanical planarization (CMP)).

Step P+4 through Step Q: Perform the contact mask through thepassivation deposition and the pad etching steps on the first side ofthe device wafer. At this point, the wafer has completed conventionalwafer processing.

Step Q+1: Do nothing, apply tape to one side of the wafer, or mount thewafer on a substrate.

Step Q+2: Remove handle wafer 2 from the second side of the devicewafer, resulting in a structure like that of e.g. FIG. 16E.

Step Q+3 through end: Plate one or both sides of the device. Finalprocessing continues through dicing (chip separation) and testing asappropriate.

In one sample embodiment, the first side of the device can be platedbefore handle wafer 2 is removed from the second side of the devicewafer.

In one sample embodiment, BTRAN fabrication can start withdouble-side-polished starting wafers. Following the high-temperaturebonding step, alignment marks can be placed on both exposed surfaces ofthe bonded wafer stack using an alignment algorithm. In another sampleembodiment, front-surface-to-back-surface alignment can be obtained by,e.g., infrared alignment, which allows the features on one wafer surfaceto be “seen” through the wafer during alignment. In yet another sampleembodiment, front-back alignment can be obtained by mechanical means,e.g. aligning to the first surface of the wafer while the mask ispresent on the second surfaces. Each of these front-back alignmenttechniques has advantages and disadvantages, including associatedequipment costs.

In one sample BTRAN fabrication embodiment, double-sided plating of aBTRAN can use the same metal and pad pattern on each surface. Connectionto the desired regions on each surface can then be made using apatterned layer. In one sample embodiment, contact can be made to thebottom surface using e.g. a patterned metallization on a ceramic, and tothe top surface using e.g. a patterned copper lead frame. The ability touse the same pad mask on both surfaces can greatly simplify fabrication.

In one sample embodiment, contact to both the bottom and the top of thedie is preferably obtained by a layer of solder. The solder is typicallydeposited on the patterned regions which have also been plated, andwhich are present on each surface of the die.

However, it can be advantageous from a manufacturing perspective toattach the bottom of a BTRAN die to a metallized ceramic using solder,and to bond large-diameter wire to the metallized regions on the top ofthe die.

One complicating factor, however, is that large regions of platedmaterial, e.g. nickel, have residual stress that can crack or otherwisedamage thin die.

The present application teaches inter alia that the same metal and padmasks can be used on both top and bottom surfaces of a BTRAN, subject tothree conditions:

1) There are sufficient bonding pads of an appropriate size on the topsurface of the die to accommodate the necessary number of large-diameterwire bonds needed.

2) There are enough plated regions on the bottom of the die to allow theformation of a low resistance contact.

3) The pattern of open plated regions does not result in stress largeenough to damage the die.

Accordingly, pad masks are proposed for BTRAN fabrication which have amixture of (typically relatively large) open regions suitable for bothwire bonds and a plated layer, and smaller open regions which can beplated but generally not bonded.

One sample embodiment of a pad mask for BTRAN fabrication according tothe present inventions can be seen in, e.g., FIG. 17. In FIG. 17, largebonding pads 1740 can be used e.g. for plating or wire bonding. In onesample embodiment, large bonding pads 1740 can be, e.g., 80 mils by 30mils, and can be spaced e.g. 12 mils apart. Small bonding pads 1738 canfill in the surrounding area not taken by large bonding pads 1740, andcan be used e.g. for plating. In one sample embodiment, large bondingpads 1740 in left chip region 1742 can be offset from large bonding pads1740 in right chip region 1744 to accommodate large-diameter wire on allpads.

FIGS. 18A-18B show another sample embodiment of BTRAN fabricationaccording to the present inventions. In FIG. 18A, a sample BTRAN padmask is shown overlaid on a sample metallization, while in FIG. 18B,only the sample pad mask is shown.

The design of the edge termination structure of a high voltagesemiconductor device such as a BTRAN is crucial to its long termoperation. A termination structure designed to operate in a given set ofconditions can exhibit a considerable reduction in voltage handlingability in the presence of unwanted positive or negative charge at ornear the surface of the termination structure. A considerable decreasein breakdown voltage can occur in the presence of either positive ornegative charge in the termination region (as demonstrated in “TheEffect of Static and Dynamic Parasitic Charge in the Termination Area ofHigh Voltage Devices and Possible Solutions”, T. Trajkovic et al., whichis hereby incorporated by reference).

The present innovative BTRAN fabrication methods also teach (inter alia)the use of novel and innovative structures to prevent any decrease inthe breakdown voltage of a device by the presence of charge in thetermination region. One sample embodiment of these innovativestructures, as seen in e.g. FIG. 19, can consist of a P+ region thatcontains one or more n-type regions.

This combination of two doped regions, one within the other, is similarto the body and source of a DMOS transistor, or to the base and emitterof a bipolar transistor.

In one sample embodiment, a BTRAN termination structure according to thepresent inventions can be fabricated through the use of two masks, onefor each dopant type. In another sample embodiment, a BTRAN terminationstructure according to the present inventions can be fabricated by usinga single mask with both dopant types introduced through the sameopening. In each of these cases, the required openings can be added topre-existing masking layers, with the final dopant distributions beingobtained using existing dopant-introduction and diffusion steps.

In some sample embodiments, innovative bidirectional devices of thepresent inventions can have pad metallizations which are not symmetricbetween the front and back of a device, as in e.g. FIG. 20A. FIG. 20Bshows how offset contact pads can significantly improve thermaldissipation by significantly reducing the distance from any point in thedie to a region with low thermal resistance. In one sample embodiment,contact pads on each side can be spaced e.g. 1270 μm apart (in the xdirection), and the die can be e.g. 100-200 μm thick (in the zdirection). In one sample embodiment, contact pads on each side of thedie can be offset to minimize distance to regions with low thermalresistance. FIGS. 20C-20D show alternative embodiments of padmetallization fractions.

Advantages

The disclosed innovations, in various embodiments, provide one or moreof at least the following advantages. However, not all of theseadvantages result from every one of the innovations disclosed, and thislist of advantages does not limit the various claimed inventions.

-   -   On-state voltage drop is less than a diode drop.    -   Bidirectional operation is achieved with identical electrical        characteristics in either direction.    -   Provides totally flat planar device.    -   Bidirectional bipolar transistors operate under conditions of        high non-equilibrium carrier concentration.    -   High voltage resistance.    -   High ruggedness.    -   Innovative fabrication techniques enable two-sided bidirectional        device fabrication.    -   Fully bidirectional conduction with less than a diode drop in        each direction.    -   Enables fully-bidirectional switching in power-packet-switching        power converters.    -   No double-diffused base necessary.

According to some but not necessarily all embodiments, there isprovided: Methods, systems, circuits, and devices forpower-packet-switching power converters using bidirectional bipolartransistors (BTRANs) for switching Four-terminal three-layer BTRANsprovide substantially identical operation in either direction withforward voltages of less than a diode drop. BTRANs are fully symmetricmerged double-base bidirectional bipolar opposite-faced devices whichoperate under conditions of high non-equilibrium carrier concentration,and which can have surprising synergies when used as bidirectionalswitches for power-packet-switching power converters. BTRANs are driveninto a state of high carrier concentration, making the on-state voltagedrop very low.

According to some but not necessarily all embodiments, there isprovided: A power-packet-switching power converter, comprising: aplurality of phase legs which each include two bidirectional switcheswhich can connect a respective external line to either side of a linkinductor which is paralleled by a capacitor, each said bidirectionalswitch comprising: first and second first-conductivity-type emitterregions on opposing faces of a second-conductivity-type semiconductormass, and first and second second-conductivity-type base contactregions, in proximity to said first and second emitter regionsrespectively; control circuitry which, repeatedly, turns on a selectedone or two of said bidirectional switches to drive energy from one ormore input lines into said inductor, and then turns off all of saidswitches to disconnect said inductor, and then turns on a differentselected one or two of said bidirectional switches to drive energy fromsaid inductor onto one or two output lines; and a drive circuit which,when the control circuitry selects one of said bidirectional switchesfor turn-on, drives a base contact region of that switch, to forwardbias the associated emitter-base junction and permit majority carriersto flow to the other emitter region on the opposing surface; wherein thedrive circuit applies sufficient current to the selected base contactregion to generate a nonequilibrium carrier concentration, in theinterior of said semiconductor mass, which is more than thirty times asgreat as the off-state equilibrium majority carrier concentration in thesemiconductor mass, to thereby lower the voltage drop across the switch.

According to some but not necessarily all embodiments, there isprovided: A power-packet-switching power converter, comprising: aplurality of phase legs which each include two bidirectional switcheswhich can connect a respective external line to either side of a linkinductor which is paralleled by a capacitor, each said bidirectionalswitch comprising: first and second first-conductivity-type emitterregions on opposing faces of a second-conductivity-type semiconductormass, and first and second second-conductivity-type base contactregions, in proximity to said first and second emitter regionsrespectively; control circuitry which, repeatedly, turns on a selectedone or two of said bidirectional switches to drive energy from one ormore input lines into said inductor, and then turns off all of saidswitches to disconnect said inductor, and then turns on a differentselected one or two of said bidirectional switches to drive energy fromsaid inductor onto one or two output lines; and a drive circuit which,when the control circuitry selects one of said bidirectional switchesfor turn-on, drives a base contact region of that switch, to forwardbias the associated emitter-base junction and permit majority carriersto flow to the other emitter region on the opposing surface; wherein thedrive circuit applies sufficient current to the selected base contactregion to drive the beta down to less than one-quarter of itssmall-signal value.

According to some but not necessarily all embodiments, there isprovided: A power-packet-switching power converter, comprising: aplurality of phase legs which each include two bidirectional switcheswhich can connect a respective external line to either side of a linkinductor which is paralleled by a capacitor, each said bidirectionalswitch comprising: first and second first-conductivity-type emitterregions on opposing faces of a second-conductivity-type semiconductormass, and first and second second-conductivity-type base contactregions, in proximity to said first and second emitter regionsrespectively; control circuitry which, repeatedly, turns on a selectedone or two of said bidirectional switches to drive energy from one ormore input lines into said inductor, and then turns off all of saidswitches to disconnect said inductor, and then turns on a differentselected one or two of said bidirectional switches to drive energy fromsaid inductor onto one or two output lines; and a drive circuit which,when the control circuitry selects one of said bidirectional switchesfor turn-on, drives a base contact region of that switch, to forwardbias the associated emitter-base junction and permit majority carriersto flow to the other emitter region on the opposing surface; wherein thedrive circuit applies sufficient current to the selected base contactregion to generate a nonequilibrium carrier concentration, in theinterior of said semiconductor mass, which is more than thirty times asgreat as the off-state equilibrium majority carrier concentration, tothereby lower the voltage drop across the switch to less than half adiode drop.

According to some but not necessarily all embodiments, there isprovided: A power-packet-switching power converter, comprising: aplurality of phase legs which each include two bidirectional switcheswhich can connect a respective external line to either side of a linkinductor which is paralleled by a capacitor; each said bidirectionalswitch comprising: first-conductivity-type emitter regions on opposingfaces of a second-conductivity-type semiconductor mass, andsecond-conductivity-type base contact regions in proximity to respectiveemitter regions; control circuitry which turns on two of saidbidirectional switches to drive energy from one or more input lines intosaid inductor, and then turns off all of said switches to disconnectsaid inductor, and then turns on a different two of said bidirectionalswitches to drive energy from said inductor onto one or two outputlines; and a drive circuit which, when the control circuitry selects oneof said bidirectional switches for turn-on: begins turn-on by shorting afirst one of the base contact regions of that switch to the respectiveemitter region, while leaving the base contact regions on the opposingface of that switch floating; drives said first one of the base contactregions, to forward bias the associated emitter-base junction and permitmajority carriers flow to the other emitter region on the opposingsurface, thereby entering saturation mode; shorts said first one of thebase contact regions to the respective emitter region, thereby exitingsaturation mode; begins turn-off by shorting the base contact region onthe opposing surface to the respective emitter region; and completesturn-off by causing said first one of the base contact region to float;wherein the drive circuit applies sufficient current to the selectedbase contact region to generate a nonequilibrium carrier concentration,in said semiconductor mass, to thereby lower the voltage drop across theswitch.

According to some but not necessarily all embodiments, there isprovided: A bidirectional power switching circuit, comprising: first andsecond first-conductivity-type emitter regions on opposing faces of asecond-conductivity-type semiconductor mass, and first and secondsecond-conductivity-type base contact regions, in proximity to saidfirst and second emitter regions respectively; and a drive circuitwhich, when the control circuitry selects one of said bidirectionalswitches for turn-on, drives a base contact region of that switch, toforward bias the associated emitter-base junction and permit majoritycarriers to flow to the other emitter region on the opposing surface;wherein the drive circuit applies sufficient current to the selectedbase contact region to generate a nonequilibrium carrier concentration,in the interior of said semiconductor mass, which is more than thirtytimes as great as the off-state equilibrium majority carrierconcentration, to thereby lower the voltage drop across the switch.

According to some but not necessarily all embodiments, there isprovided: A method of operating a power-packet-switching powerconverter, comprising: driving a first array of bidirectional switchesto drive power into a link inductor which is paralleled by a capacitor;driving a second array of bidirectional switches to draw power onto anoutput line from said inductor; wherein each said bidirectional switchcomprises first-conductivity-type emitter regions on opposing faces of asecond-conductivity-type base region, and second-conductivity-type basecontact regions on said opposing faces of said second-conductivity-typebase region; wherein driving each said bidirectional switch comprises:when one said face of one said bidirectional switch is desired to be inpassive off mode, clamping a voltage of the respective base contactregion to be less than or equal to a voltage of the respective emitterregion plus a Schottky diode drop; when one said face of one saidbidirectional switch is desired to be in active off mode or in diodemode, shorting the respective base contact region to the respectiveemitter region; and when one said face of one said bidirectional switchis desired to be in active on mode, injecting minority charge carriersinto the respective base contact region.

According to some but not necessarily all embodiments, there isprovided: A semiconductor device, comprising: first-conductivity-typeemitter regions on opposing faces of a second-conductivity-typesemiconductor mass; second-conductivity-type base contact regions onsaid opposing faces of said second-conductivity-type semiconductor mass;wherein said first-conductivity-type emitter regions and saidsecond-conductivity-type semiconductor mass form a heterojunctiontherebetween; drive circuitry which applies sufficient current to theselected base contact region to generate a nonequilibrium carrierconcentration, in the interior of said semiconductor mass, which is morethan thirty times as great as the off-state equilibrium majority carrierconcentration, to thereby lower the voltage drop.

According to some but not necessarily all embodiments, there isprovided: A semiconductor device, comprising: first-conductivity-typeemitter regions on opposing faces of a second-conductivity-typesemiconductor mass; second-conductivity-type base contact regions onsaid opposing faces of said second-conductivity-type semiconductor mass;a thin layer of tunnel oxide between each said first-conductivity-typeemitter region and said second-conductivity-type semiconductor mass, andwhich forms a differential between holes and electrons; drive circuitrywhich applies sufficient current to the selected base contact region togenerate a nonequilibrium carrier concentration, in the interior of saidsemiconductor mass, which is more than thirty times as great as theoff-state equilibrium majority carrier concentration, to thereby lowerthe voltage drop.

According to some but not necessarily all embodiments, there isprovided: A semiconductor device, comprising: first-conductivity-typeemitter regions on opposing faces of a second-conductivity-typesemiconductor mass; second-conductivity-type base contact regions onsaid opposing faces of said second-conductivity-type semiconductor mass;a thin layer of tunnel oxide between each said first-conductivity-typeemitter region and a respective emitter metallization, and which forms adifferential between holes and electrons; drive circuitry which appliessufficient current to the selected base contact region to generate anonequilibrium carrier concentration, in the interior of saidsemiconductor mass, which is more than thirty times as great as theoff-state equilibrium majority carrier concentration, to thereby lowerthe voltage drop.

Modifications and Variations

As will be recognized by those skilled in the art, the innovativeconcepts described in the present application can be modified and variedover a tremendous range of applications, and accordingly the scope ofpatented subject matter is not limited by any of the specific exemplaryteachings given. It is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims.

In some embodiments, oxide trenches 1330 can be present between sourceregions and adjacent base contact regions. In other embodiments, thiscan be different.

In some heterojunction embodiments, emitter/collector regions can beamorphous silicon on a crystalline silicon substrate. In otherembodiments, heterojunctions can be formed differently.

In most presently-preferred embodiments, base contact regions are moreheavily doped than the majority of the base. However, in otherembodiments, this can be different.

In some heterojunction sample embodiments, heterojunctions can beprovided by junctions between e.g. amorphous and crystalline silicon. Inother embodiments, heterojunctions can be e.g. provided by differentmaterials. In still other embodiments, heterojunctions can be providedby e.g. crystalline-crystalline junctions, between e.g. crystallinesilicon and a different crystalline semiconductor, provided e.g. thatthe resultant junction potentials are sharp enough to increase injectionefficiency.

In one contemplated alternative embodiment, smaller emittermetallization fractions can be used to reduce hole recombination andthereby increase injection efficiency.

In some sample embodiments of heterojunction BTRANs, heterojunctionemitter/collector regions can be e.g. amorphous silicon. In other sampleembodiments, heterojunction emitter/collector regions can be e.g.polycrystalline silicon. In still other embodiments, this can bedifferent.

In some embodiments, one base can be driven more than the other base.

In some embodiments, other methods for increasing injection efficiencyin high level non-equilibrium carrier densities can be used, singly orin combination with the innovative techniques disclosed herein.

In some alternative embodiments, the innovative BTRAN fabricationtechniques described herein can be applied to other two-sidedbidirectional devices, such as e.g. bidirectional IGBTs.

In some alternative embodiments, the innovative devices of the presentinventions can be advantageously applied to different sorts of powerconverters. In some alternative embodiments, the present innovativedevices can be used for e.g. matrix converters. In other alternativeembodiments, the present innovative devices can be applied to e.g.voltage reduction regulators as used for induction motor efficiencyoptimization and soft start.

In some alternative embodiments, emitter regions can be recessedemitters located in trenches. In other alternative embodiments, basecontact regions can be recessed base contact regions located intrenches.

In some alternative embodiments, field-shaping regions can be presentbelow base contact regions or emitter regions or both.

Additional general background, which helps to show variations andimplementations, can be found in the following publications, all ofwhich are hereby incorporated by reference: “The Effect of Static andDynamic Parasitic Charge in the Termination Area of High Voltage Devicesand Possible Solutions”, T. Trajkovic et al.

Additional general background, which helps to show variations andimplementations, as well as some features which can be implementedsynergistically with the inventions claimed below, may be found in thefollowing US patent applications. All of these applications have atleast some common ownership, copendency, and inventorship with thepresent application, and all of them, as well as any material directlyor indirectly incorporated within them, are hereby incorporated byreference: U.S. Pat. No. 8,406,265, U.S. Pat. No. 8,400,800, U.S. Pat.No. 8,395,910, U.S. Pat. No. 8,391,033, U.S. Pat. No. 8,345,452, U.S.Pat. No. 8,300,426, U.S. Pat. No. 8,295,069, U.S. Pat. No. 7,778,045,U.S. Pat. No. 7,599,196; US 2012-0279567 A1, US 2012-0268975 A1, US2012-0274138 A1, US 2013-0038129 A1, US 2012-0051100 A1; PCT/U514/16740,PCT/US14/26822, PCT/U514/35954, PCT/U514/35960; Ser. Nos. 14/182,243,14/182,236, 14/182,245, 14/182,246, 14/183,403, 14/182,249, 14/182,250,14/182,251, 14/182,256, 14/182,268, 14/183,259, 14/182,265, 14/183,415,14/182,280, 14/183,422, 14/182,252, 14/183,245, 14/183,274, 14/183,289,14/183,309, 14/183,335, 14/183,371, 14/182,270, 14/182,277, 14/207,039,14/209,885, 14/260,120, 14/265,300, 14/265,312, 14/265,315; U.S.Provisionals 61/765,098, 61/765,099, 61/765,100, 61/765,102, 61/765,104,61/765,107, 61/765,110, 61/765,112, 61/765,114, 61/765,116, 61/765,118,61/765,119, 61/765,122, 61/765,123, 61/765,126, 61/765,129, 61/765,131,61/765,132, 61/765,137, 61/765,139, 61/765,144, 61/765,146 all filedFeb. 15, 2013; 61/778,648, 61/778,661, 61/778,680, 61/784,001 all filedMar. 13, 2013; 61/814,993 filed Apr. 23, 2013; 61/817,012, 61/817,019,61/817,092 filed Apr. 29, 2013; 61/838,578 filed Jun. 24, 2013;61/841,618, 61/841,621, 61/841,624 all filed Jul. 1, 2013; 61/914,491and 61/914,538 filed Dec. 11, 2013; 61/924,884 filed Jan. 8, 2014;61/925,311 filed Jan. 9, 2014; 61/928,133 filed Jan. 16, 2014;61/928,644 filed Jan. 17, 2014; 61/929,731 and 61/929,874 filed Jan. 21,2014; 61/931,785 filed Jan. 27, 2014; 61/932,422 filed Jan. 28, 2014;61/933,442 filed Jan. 30, 2014; 62/007,004 filed Jun. 3, 2014; and allpriority applications of any of the above thereof, each and every one ofwhich is hereby incorporated by reference.

None of the description in the present application should be read asimplying that any particular element, step, or function is an essentialelement which must be included in the claim scope: THE SCOPE OF PATENTEDSUBJECT MATTER IS DEFINED ONLY BY THE ALLOWED CLAIMS. Moreover, none ofthese claims are intended to invoke paragraph six of 35 USC section 112unless the exact words “means for” are followed by a participle.

The claims as filed are intended to be as comprehensive as possible, andNO subject matter is intentionally relinquished, dedicated, orabandoned.

What is claimed is:
 1. A switching circuit comprising: a two-basebidirectional npn semiconductor switching device which includes both ann-type emitter/collector region and also a p-type base contact region oneach of both first and second opposed surfaces of a p-type monolithicsemiconductor die; driving circuitry controlling said switching device,comprising control circuitry which is connected separately to the basecontact regions on the first and second opposed surfaces; and first andsecond distinct voltage-limiting circuits, each comprising a seriescombination of a low-voltage diode and a normally-on switch, eachvoltage-limiting circuit connected to one of the first and secondopposed surfaces so that, when the normally-on switch is on, the anodeof the low-voltage diode is connected to the p-type base contact region,and the cathode of the low-voltage diode is connected to the n-typeemitter/collector region; wherein the low-voltage diode turns on at aforward voltage which is less than the diode drop of the p-n junctionbetween an emitter/collector region and the semiconductor die; whereby,while the normally-on switch is ON, the p-n junction between anemitter/collector region and the semiconductor die cannot ever operateunder forward bias.
 2. The switching circuit of claim 1, wherein thenormally-on switch is connected to the control circuitry.
 3. Theswitching circuit of claim 1, wherein the normally-on switch is a JFET.4. The switching circuit of claim 1, wherein the normally-on switch is aJFET having a gate connected to the control circuitry.
 5. The switchingcircuit of claim 1, wherein the die is silicon.
 6. The switching circuitof claim 1, wherein the low-voltage diode is a schottky barrier diode.7. The switching circuit of claim 1, wherein the die is silicon, and thelow-voltage diode is a schottky barrier diode.
 8. The switching circuitof claim 1, wherein the emitter/collector region on the first surface isnot directly electrically connected to the emitter/collector region onthe second surface, except through the semiconductor die itself.
 9. Theswitching circuit of claim 1, wherein the base contact region on thefirst surface is not directly electrically connected to the base contactregion on the second surface, except through the semiconductor dieitself.
 10. A switching circuit comprising: a double-base bipolartransistor, having a distinct n-type emitter/collector region on each ofboth first and second opposite surfaces of a p-type semiconductor die,each emitter/collector region defining a p-n emitter junction withrespect to the die; first and second voltage-limiting circuits connectedto said first and second opposite surfaces, wherein each saidvoltage-limiting circuit includes a diode and a normally-on switch inseries, with the anode of the diode operatively connected to the p sideof the respective emitter junction, and the cathode operativelyconnected to the n side of the respective emitter junction; wherein thediodes in said voltage-limiting circuits have respective forward diodevoltage drops which are substantially less than the forward diodevoltage drop characteristic of the respective p-n emitter junction. 11.The switching circuit of claim 10, wherein the normally-on switch is aJFET.
 12. The switching circuit of claim 10, wherein the die is silicon.13. The switching circuit of claim 10, wherein the diode is a schottkybarrier diode.
 14. The switching circuit of claim 10, wherein the die issilicon, and the diode is a schottky barrier diode.
 15. The switchingcircuit of claim 10, wherein the emitter/collector region on the firstsurface is not directly electrically connected to the emitter/collectorregion on the second surface.
 16. A switching circuit comprising: adouble-base bipolar transistor, having a distinct p-typeemitter/collector region on each of both first and second oppositesurfaces of an n-type semiconductor die, each emitter/collector regiondefining a p-n emitter junction with respect to the die; first andsecond voltage-limiting circuits connected to said first and secondsurfaces, wherein each voltage-limiting circuit includes a diode and anormally-on switch in series, with the anode of the diode operativelyconnected to the p side of the respective emitter junction, and thecathode operatively connected to the n side of the respective emitterjunction; wherein the diodes in said voltage-limiting circuits haverespective forward diode voltage drops which are substantially less thanthe forward diode voltage drop characteristic of the respective p-nemitter junction.
 17. The switching circuit of claim 16, wherein thenormally-on switch is a JFET.
 18. The switching circuit of claim 16,wherein the die is silicon.
 19. The switching circuit of claim 16,wherein the diode is a schottky barrier diode.
 20. The switching circuitof claim 16, wherein the die is silicon, and the diode is a schottkybarrier diode.